Recombinant vaccines are occasionally employed in human and veterinary medicine as an alternative to more traditional approaches based on killed or attenuated pathogens. Many foreign antigens delivered systemically to the body in this way are capable of activating only one arm of the immune system, by stimulating the humoral immune response to generate antibodies by the Major Histocompatibility Complex (MHC) class II pathway. However, an ideal vaccine should also induce a cellular response by destruction of infected cells through activation of the MHC class I pathway. The latter response is achieved through cytosolic degradation of foreign protein in infected cells, such that fragments of the foreign material are associated with MHC class I molecules and shuttled to the cell surface for presentation to CD8+ cytotoxic T cells (CTL).
Nucleic acid vaccines (NAVs) are a relatively new form of technology which are useful for delivery of pathogen antigens, especially viral antigens. As the viral proteins encoded by the vaccines are expressed in situ by the host's cellular apparatus, theory suggests that they should elicit a cell-mediated immune response capable of protecting animals when challenged. Results, however, have been mixed: in fish, NAVs expressing the infectious haematopoietic necrosis virus (IHNV) G protein (surface glycoprotein), and the viral haemorrhagic septicaemia virus G protein are effective against. IHNV and VHSV infections, respectively. However, it has been difficult to demonstrate convincing protection of fish using NAVs based on other viral antigens.
It is an object of the present invention to provide improved vaccines effective against a variety of diseases in fish and other animals caused by infection with pathogens.